Switch from QGLWidget to QOpenGLWidget

* QGLWidget has been deprecated => Nuke it and don't link Qt5::OpenGL

* Switch to the better, newer QOpenGLWidget (introduced in Qt 5.4)

* Fixes the OpenGL Renderer not working

BUG: 403323
BUG: 410083
BUG: 420672


FIXED-IN: 20.12.3

Signed-off-by: Madhav Kanbur <abcdjdj@gmail.com>